In various electronic circuits it is desirable to monitor an input signal to determine if the input signal is behaving in a predetermined manner. If the input signal is not behaving properly an output signal is generated. Such circuits are generally referred to as "watchdog" circuits. Specifically, in electronic postage meters it is desirable to monitor the output signal from a microprocessor to determine that it is toggling at a predetermined minimum frequency for proper activation of various components in the electronic postage meter, e.g., non-volatile memory (NVM).
Various electronic postage meter systems employing microprocessors have been developed, see, for example, the systems disclosed in U.S. Pat. No. 3,978,457 for Microcomputerized Electronic Postage Meter Systems, U.S. Pat. No. 3,938,095 for Computer Responsive Postage Meter, European patent application No. 80400603.9, filed May 5, 1980, which corresponds to U.S. Pat. No. 4,484,307 for Electronic Postage Meter Having Improved Security and Fault Tolerance Features, corresponding to U.S. Pat. No. 4,301,507, for Electronic Postage Meter Having Plural Computing Systems, and copending application Ser. No. 447,815, filed Dec. 8, 1982, for Stand-Alone Electronic Mailing Machine now U.S. Pat. No. 4,579,054.
Generally electronic postage meters include some form of NVM capability to store critical postage accounting information. This information includes, for example, the amount of postage remaining in the meter for subsequent printing and the total amount of postage already printed by the meter. Other types of accounting or operating data may also be stored in the non-volatile memory, as desired.
However, conditions can occur in electronic postage meters where information stored in NVM may be lost. A total line power failure or fluctuation in voltage conditions can cause the microprocessor associated with the meter to operate erratically and either cause erasure of data or the writing of spurious data in the NVM. The erasure of data or the writing of spurious data in the NVM may result in a loss of critical accounting information. Since the accounting data changes with the printing of postage and is not permanently stored elsewhere, there is no way to recapture or reconstruct the lost accounting information. Under such circumstances, it is possible that a user may suffer a loss of postage funds.
For example, the output from the microprocessor may be functioning erratically after the microprocessor is first released from its reset condition. This may cause errors in operation of the postage meter and/or the erroneous storage or erasure of data in NVM. If the output from the microprocessor is not behaving in a predetermined manner, it is desirable to signal and/or reset the microprocessor.
Various watchdog circuits are known for use in electronic postage meters. See for example, patent application Ser. No. 447,750, filed on Dec. 8, 1982 for Monitoring Circuit For An Electronic Postage Meter, now U.S. Pat. No. 4,600,987 which discloses a timing means for generating an output signal in the absence of detection of an output signal from a microprocessor within a predetermined time interval, and the aforementioned European patent application No. 80400603.9 which discloses two redundant circuits composed of multivibrators and flip-flops which are set to a fault condition based on the absence of signals from a microprocessor.